Surface layers or zones of silicon, when doped with such materials as phosphorus or boron, produce areas of differing electron population commonly referred to as p and n layers. The familiar p-n junctions between such layers or zones form the basis of many well known semiconductor devices. Recently, efforts have been initiated to utilize this silicon technology in the production of photovoltaic devices known as solar cells. Such cells are activated by solar radiation to generate electrical energy, thereby providing a means of converting solar energy into electrical energy.
Silicon base semiconductor devices have generally been very small in size with primary emphasis being placed on electrical performance and miniaturization of the device. Accordingly, while reduction in material cost is rarely ever ignored, this economic factor has received relatively little consideration in the production of semiconductors.
Currently, however, the successful development of an operative solar cell based on doped layers of silicon promises to require a much larger structure wherein material cost will be a controlling factor. Accordingly, attention is being directed to photovoltaic devices wherein a thin silicon film is deposited on a substrate that is less expensive to produce than silicon. Such a substrate must of course be chemically, electrically, and physically compatible with silicon to produce an operable device. However, subject to this basic requirement, a low substrate cost, without additional film forming or processing costs, is now becoming a prime consideration. Specifically, it would be most desirable to provide a substrate material that is substantially less expensive than silicon itself, the primary substrate presently used in semiconductor devices and integrated circuitry.
A major factor contributing to material cost is the cost of forming the material into a relatively large body, more especially in the form of a sheet. Only a few materials have the potential of being inexpensively formed in this manner as well as the potential of compatibility with a silicon film. These materials include glasses, glass-ceramics, and polycrystalline ceramics such as sintered alumina.
Various methods are disclosed in the prior art for producing thin silicon films on the order of 20 to 100 microns in thickness. Such methods are directed to depositing the film as a separate entity on a substrate, rather than the doping of an integral surface layer on a body of silicon as is the common semiconductor commercial practice.
In accordance with a method of particular interest, the vapor of a silicon compound such as silicon hydride (SiH.sub.4) is pyrolyzed in contact with a substrate material while at a temperature in the range of 700.degree.-1300.degree. C. to produce silicon which is deposited as a film on the substrate.
It is vitally necessary that the substrate material employed be chemically, electrically and physically compatible with the silicon film deposited thereon, as well as chemically inert with respect to the materials used in the process. It is particularly important that the linear thermal expansion coefficient of the substrate material closely match that of silicon over the entire temperature range from the deposition temperature down to room temperature. Otherwise, warping of the substrate and/or development of stresses in the silicon film tend to occur and produce subsequent erratic electrical behavior in the device being produced.
It is obvious that there should be no reaction between the silicon film being produced and the substrate surface upon which it is being deposited. It is also necessary that other materials employed in the process, in particular the silicon compound, the products of decomposition, and the carrier gas, should be chemically unreactive with the substrate. Finally, there should be no inherent or induced crystallization tendencies in the substrate material which would tend to effect the uniformity of the silicon film.